dmvianna - 1 year ago 277
R Question

# Simpler population pyramid in ggplot2

I want to create a population pyramid with ggplot2. This question was asked before, but I believe the solution must be far simpler.

``````test <- (data.frame(v=rnorm(1000), g=c('M','F')))
require(ggplot2)
ggplot(data=test, aes(x=v)) +
geom_histogram() +
coord_flip() +
facet_grid(. ~ g)
``````

Produces this image. In my opinion, the only step missing here to create a population pyramid is to invert the x axis of the first facet, so that is goes from 50 to 0, while keeping the second untouched. Can anyone help?

Here is a solution without the faceting. First, create data frame. I used values from 1 to 20 to ensure that none of values is negative (with population pyramids you don't get negative counts/ages).

``````test <- data.frame(v=sample(1:20,1000,replace=T), g=c('M','F'))
``````

Then combined two `geom_bar()` calls separately for each of `g` values. For `F` counts are calculated as they are but for `M` counts are multiplied by -1 to get bar in opposite direction. Then `scale_y_continuous()` is used to get pretty values for axis.

``````require(ggplot2)
require(plyr)
ggplot(data=test,aes(x=as.factor(v),fill=g)) +
geom_bar(subset=.(g=="F")) +
geom_bar(subset=.(g=="M"),aes(y=..count..*(-1))) +
scale_y_continuous(breaks=seq(-40,40,10),labels=abs(seq(-40,40,10))) +
coord_flip()
``````

## UPDATE

As argument `subset=.` is deprecated in the latest `ggplot2` versions the same result can be atchieved with function `subset()`.

``````ggplot(data=test,aes(x=as.factor(v),fill=g)) +
geom_bar(data=subset(test,g=="F")) +
geom_bar(data=subset(test,g=="M"),aes(y=..count..*(-1))) +
scale_y_continuous(breaks=seq(-40,40,10),labels=abs(seq(-40,40,10))) +
coord_flip()
``````

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